Method for Recovering Indium

ABSTRACT

There is provided a method for recovering indium, the method being capable of recovering indium having a high purity at simple and inexpensive steps in a short time and with a high recovery. After indium containing substances, such as ITO target scraps, are crushed, the crushed substances are ground until the percentage of coarse particles having a larger particle size than a predetermined particle size is not larger than a predetermined percentage. Thereafter, the ground substances are dissolved in an acid, and the solution thus obtained is neutralized with an alkali so that the pH of the solution is 0.5 to 4. Then, the solution is aged at a temperature of 60 to 70° C. for 3 hours or longer to deposit and remove hydroxides of predetermined metal ions in the solution. Then, hydrogen sulfide gas is blown into the solution to deposit and remove sulfides of metal ions which are harmful to electrolysis at the subsequent step, and thereafter, the solution thus obtained is used as an electrolytic solution for electrowinning indium metal to recover indium having a high purity.

TECHNICAL FIELD

The present invention relates generally to a method for recovering indium. More specifically, the invention relates to a method for recovering indium metal from indium containing substances, such as indium-tin-oxide (ITO) target scraps.

BACKGROUND ART

With the rapid development of liquid crystal techniques in recent years, a demand for ITO films used as transparent conductive films for liquid crystals is remarkably increased, and the quantity of ITO target materials consumed as the raw materials of ITO films is also remarkably increased.

As a conventional method for recovering indium from indium containing substances, such as ITO target scraps, a method utilizing solvent extraction is proposed (see, e.g., Japanese Patent Laid-Open No. 8-91838). However, this method has complicated steps due to the repetition of extraction and back extraction, and costs a lot due to the use of expensive solvents. There is also proposed a method comprising the steps of: dissolving an indium containing substance in hydrochloric acid or a mixed acid of hydrochloric acid and sulfuric acid; putting an indium metal plate in the solution after dissolution, to replace impurity ions in the solution to deposit and remove substitution products; and electrowinning indium metal using the solution after dissolution as an electrolytic solution (see, e.g., Japanese Patent Laid-Open No. 10-204673). This method has simple steps, but there is a limit to the concentration of impurity ions capable of being removed by substitution and deposition, so that the purity of obtainable indium metal is only about 99.99%.

In order to solve such problems in conventional methods for recovering indium, there is proposed a method comprising the steps of: dissolving an indium containing substance in hydrochloric acid; adding an alkali to the solution after dissolution, to neutralize the solution so that the pH of the solution is 0.5 to 4; depositing and removing hydroxides of predetermined metal ions in the solution; blowing hydrogen sulfide gas into the solution to deposit and remove sulfides of metal ions which are harmful to electrolysis; and then, electrowinning indium metal using the solution as an initial electrolytic solution (see, e.g., Japanese Patent Laid-Open No. 2000-169991). This method is very useful as a method for recycling ITO target scraps and so forth since it is possible to recover indium having a high purity of not less than 99.999% at simple and inexpensive steps.

However, the content of indium in indium containing substances, such as ITO target scraps, and the shape and size of the substances are not constant, so that such indium containing substances can not suitably used as recycled materials in the method disclosed in Japanese Patent Laid-Open No. 2000-169991, as they are. In particular, at the step of dissolving indium containing substances in hydrochloric acid, the variation in sharp and size of raw materials causes the increase of the treating time and/or the decrease of the leaching rate, so that the treating time and leaching rate at the dissolving step are changed. If the decrease of the treating time takes precedence over the improvement of the leaching rate at the dissolving step, the leaching rate is decreased, so that it is required to additionally provide a circulating or reprocessing line for improving the leaching rate. On the other hand, if the improvement of the leaching rate (recovery) takes precedence over the decrease of the treating time at the dissolving step, it takes a lot of time to obtain a desired leaching rate. Therefore, in order to industrially utilize the method disclosed in Japanese Patent Laid-Open No. 2000-169991, it is required to achieve both of the shortening of the treating time and the improvement of the leaching rate at the dissolving step so that indium can be recovered in a short time and with a high recovery. In addition, in the method disclosed in Japanese Patent Laid-Open No. 169991, the amount of tin in indium metal obtained by electrowinning is about 1 ppm, so that it is desired to further decrease the amount of tin.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a method for recovering indium, the method being capable of recovering indium having a high purity at simple and inexpensive steps in a short time and with a high recovery.

In order to accomplish the aforementioned object, the inventors have diligently studied and found that it is possible to achieve both of the shortening of a treating time and the improvement of a leaching rate at a dissolving step and it is possible to stabilize a neutralized precipitate to decrease the amount of tin in electrowon indium metal, so that it is possible to provide a method for recovering indium, the method being capable of recovering indium having a high purity at simple and inexpensive steps in a short time and with a high recovery, if an indium containing substance is dissolved in an acid after being ground until the percentage of coarse particles having a larger particle size than a predetermined particle size is not larger than a predetermined percentage and if aging is carried out at a predetermined temperature for a predetermined period of time after neutralization. Thus, the inventors have made the present invention.

According to the present invention, a method for recovering indium comprises: a grinding step of grinding an indium containing substance until the percentage of coarse particles of the indium containing substance is not larger than a predetermined percentage, the coarse particles having a larger particle size than a predetermined particle size; a dissolving step of dissolving fine particles, which are obtained at the grinding step, in an acid, such as hydrochloric acid, sulfuric acid or nitric acid; a neutralizing step of adding an alkali to a solution after dissolution, which is obtained at the dissolving step, to neutralize the solution after dissolution so that the solution after dissolution has a pH of 0.5 to 4, to deposit and remove hydroxides of predetermined metal ions in the solution after dissolution; an aging step of aging a solution after neutralization, which is obtained at the neutralizing step, at a predetermined temperature for a predetermined period of time; a sulfurizing step of blowing hydrogen sulfide gas into a solution after aging, which is obtained at the aging step, to deposit and remove sulfides of metal ions; and an electrowinning step of using a solution after sulfurization, which is obtained at the sulfurizing step, as an initial electrolytic solution to electrowin indium metal. At the grinding step in this method for recovering indium, the predetermined particle size is preferably 150 μm. The predetermined percentage is preferably 10%, more preferably 5% and most preferably substantially 0%. At the aging step, the predetermined temperature is preferably in the range of from a room temperature to 80° C., more preferably in the range of from 40° C. to 70° C., and most preferably in the range of from 60° C. to 70° C. The predetermined period of time is preferably 2 hours or longer, and more preferably 3 hours or longer. Furthermore, the method for recovering indium may further comprise a classification step of removing coarse particles having a larger particle size than the predetermined particle size by classification before the dissolving step after the grinding step. The indium containing substance is preferably an ITO target scrap.

According to the present invention, indium has a purity of not less than 99.999% and contains tin, the content of which is not larger than 0.5 ppm and preferably not larger than 0.1 ppm. Alternatively, indium is obtained by electrowinning which uses indium-tin-oxide as a raw material, and contains tin, the content of which is not larger than 0.5 ppm and preferably not larger than 0.1 ppm. Such indium can be obtained by the above described method for recovering indium according to the present invention.

According to the present invention, it is possible to recover indium having a high purity at simple and inexpensive steps in a short time and with a high recovery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process drawing showing a method for recovering indium according to the present invention;

FIG. 2 is a graph showing a variation in leaching rate with respect to time in Examples 1 and 2 and Comparative Example 1;

FIG. 3 is a graph showing a variation in leaching rate with respect to time in Example 3 and Comparative Example 2; and

FIG. 4 is a graph showing a variation in leaching rate with respect to time in Example 4 and Comparative Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the process drawing of FIG. 1, the preferred embodiment of a method for recovering indium according to the present invention will be described below.

As shown in FIG. 1, in the preferred embodiment of a method for recovering indium according to the present invention, after an indium containing substance, such as an ITO target scrap, is crushed, the crushed substance is ground until the percentage of coarse particles having a larger particle size than a predetermined particle size is not larger than a predetermined percentage. Thereafter, the ground substance is dissolved in hydrochloric acid, and an alkali, such as sodium hydroxide or sodium carbonate, is added to the solution to neutralize the solution so that the pH of the solution is 0.5 to 4. Then, the solution after dissolution is aged at a predetermined temperature for a predetermined period of time to deposit and remove hydroxides of predetermined metal ions in the solution. Then, hydrogen sulfide gas is blown into the solution to deposit and remove sulfides of metal ions which are harmful to electrolysis at the subsequent step. Thereafter, the solution thus obtained is used as an electrolytic solution for electrowinning indium metal to recover indium having a high purity.

The indium containing substances serving as raw materials are preferably ITO target scraps. In the preferred embodiment of a method for recovering indium according to the present invention, various indium containing substances may be used as the raw materials. The crushing and grinding of the raw materials may be carried out by a crusher and a grinder, respectively. The percentage of coarse particles having a larger particle size than 15 μm in fine particles obtained by grinding is preferably 10% or less, more preferably 5% or less, and most preferably substantially 0%. If the percentage of coarse particles is thus decreased by grinding, the contact of the raw materials with a hydrochloric acid solution can be improved for obtaining a desired leaching rate in a short time. Furthermore, classification, such as sieving or screening, may be carried out in order to thus remove coarse particles.

The amount of the solution after dissolving the indium containing fine particles in hydrochloric acid is preferably adjusted so that the concentration of indium is in the range of from 20 g/L to 200 g/L, and the amount of hydrochloric acid used in dissolution is preferably 1.1 to 1.5 times as large as the theoretical equivalent thereof. In addition, the solution after dissolution is preferably heated in order to promote dissolution.

The reasons why the alkali is added so that the pH is 0.5 to 4 are that the pH is adjusted to be 4 or lower to prevent the deposition of hydroxides of indium and that the pH is adjusted to be 0.5 or higher to hydrolyze impurity ions to deposit hydroxides of the ions. By thus adding the alkali, it is possible to efficiently remove most of tin which is a main impurity in ITO target scraps.

The reason why aging is carried out after neutralization is that the neutralized precipitate is stabilized. The aging temperature is preferably in the range of from a room temperature to 80° C., more preferably in the range of from 40° C. to 70° C., and most preferably in the range of from 60° C. to 70° C. The aging time is preferably 2 hours or longer, and more preferably 3 hours or longer.

After deposited hydroxides of tin and so forth are removed by filtration, hydrogen sulfide gas is blown into the filtrate to deposit and remove sulfides of copper, lead and so forth, which are harmful to electrolysis at the subsequent step, and sulfides of a very small amount of tin. The indium-dissolved solution thus purified is fed to the electrowinning step at which indium is recovered as a metal by electrowinning on appropriate electrolytic conditions. The indium metal thus recovered contains alkali metals, such as sodium, which are components of the electrolytic solution, as impurities. Therefore, the indium metal, together with solid sodium hydroxide, is heated to be mixed and melted, so that the alkali metals are dissolved in the melted sodium hydroxide to be removed, and then, the metal part obtained by gravity separation is cast in a mold to be cooled to recover indium having a high purity.

The solution after electrowinning is mixed with hydrochloric acid to be reused for the dissolution of indium containing fine particles. In order to prevent the accumulation of base metal ions, such as aluminum and iron ions, which are baser than indium, part or all of the solution after electrowinning is preferably extracted to the outside of the system.

Examples of a method for recovering indium according to the present invention will be described below in detail.

EXAMPLE 1

After ITO target scraps containing about 400 g of indium were crushed, the crushed scraps were ground to obtain fine particles wherein the percentage of coarse particles having a larger particle size than 150 μm was about 4%. The fine particles thus obtained were dissolved in 5 moles of hydrochloric acid to obtain 2L of an indium-dissolved solution. At the dissolving step using hydrochloric acid, the leaching rate was 100% after 240 minutes. After sodium hydroxide was added to the indium-dissolved solution to neutralize the solution so that the pH of the solution was 1.7, the deposited hydroxides were removed by filtration to obtain a solution after neutralization. The solution after neutralization was aged at 60° C. for 3 hours, and thereafter, hydrogen sulfide gas was brown into the solution at a flow rate of 50 cc/min for 2 minutes Then, the deposited sulfides were removed by filtration to obtain a solution after sulfurization. The solution after sulfurization thus purified was used as an initial electrolytic solution for electrowinning indium metal on electrolytic conditions (at a liquid temperature of 30° C. and a current density of 150 A/m²). The amount of tin in the electrowon indium metal was 0.4 ppm. The electrowon metal, together with solid sodium hydroxide, was heated to be mixed and melted, and a metal part obtained by gravity separation was cast in a mold to be cooled to recover cast indium. The grade of the cast indium thus obtained was not less than 99.999%, so that indium having a high purity was obtained.

The compositions of the solution after dissolution, solution after neutralization and solution after sulfurization, and the grades of the electrowon metal and cast indium, which were obtained in this example, are shown in Table 1, and the relationship between the leaching rate and time is shown in Table 2 and FIG. 2.

TABLE 1 In Sn Cu Pb Na Zn Cr Ni Fe Solution after 200 0.99 0.0025 0.0010 — — 0.0010 <0.001 0.1980 Dissolution (g/l) Solution after 198 <0.001 0.0024 0.0010 — — 0.0010 <0.001 0.0018 Neutralization (g/l) Solution after 198 <0.001 <0.001 <0.001 — — 0.0010 <0.001 <0.001 Sulfurization (g/l) Electrowon Metal — 0.4 <0.1 <0.1 31 <0.1 <0.1 0.5 <0.1 (ppm) Cast Indium >99.999 0.5 <0.1 <0.1 <0.1 <0.1 <0.1 0.3 <0.1 (ppm) (%)

TABLE 2 Comp. Example 1 Example 1 Example 2 150 μm<: 86% 150 μm<: 4% 150 μm<: 0% Time In Leaching In Leaching In Leaching (min.) (g/l) Rate (%) (g/l) Rate (%) (g/l) Rate (%) 0 0 0 0 0 0 0 30 70 35 133 66 159 79 60 120 60 179 89 192 96 90 140 70 191 95 197 98 120 146 73 195 97 201 100 180 148 74 198 99 201 100 240 151 75 200 100 201 100 360 155 77 200 100 201 100 480 160 79 201 100 201 100 600 163 81 200 100 201 100 720 164 81 200 100 201 100

Furthermore, the amount of indium in the ITO target scraps was determined by measurement using an inductively coupled plasma mass spectrometer (ICP-Mass) after the fine particles obtained by grinding the ITO target scraps were dissolved in hydrochloric acid and tartaric acid. The percentage of coarse particles (larger particles than 150 μm) in the fine particles was determined by calculating the percentage of the coarse particle part on the basis of the results that a particle size distribution was measured by a laser diffraction type particle size distribution measuring device (Model LA500 produced by HORIBA, Ltd.) after a small number of the fine particles were ultrasonically dispersed in a dispersion medium (200 mL of a sodium hexametaphosphate solution).

The compositions of the solution after dissolution, solution after neutralization and solution after sulfurization were determined by measurement using an ICP-Mass after these solutions were sampled. The grades of the electrowon metal and cast indium were determined by measurement using an ICP-Mass after each of the electrowon metal and the case indium was dissolved in nitric acid.

The variation in leaching rate with respect to time was determined by analyzing the residue and liquid, which were obtained by the solid-liquid separation of a slurry sampled every a predetermined time, by means of an ICP-Mass. The liquid after solid-liquid separation was diluted with nitric acid to be used as a sample to be analyzed. The residue after solid-liquid separation was dried to be dissolved in nitric acid and sulfuric acid, and then, dissolved in perchloric acid to be used as a sample to be analyzed.

EXAMPLE 2

Cast indium was recovered by the same method as that in Example 1, except that fine particles containing no coarse particles having a larger particle size than 153 μm were obtained by grinding. In this example, the leaching rate was 100% after 120 minutes at the dissolving step using hydrochloric acid. The amount of tin in the electrowon metal was 0.1 ppm. Furthermore, the compositions of the solution after dissolution, solution after neutralization and solution after sulfurization, and the grades of the electrowon metal and cast indium, which were obtained in this example, are shown in Table 3, and the relationship between the leaching rate and time in this example is shown in Table 2 and FIG. 2.

TABLE 3 In Sn Cu Pb Na Zn Cr Ni Fe Solution after 201 1.01 0.0026 0.0012 — — 0.0013 <0.001 0.2000 Dissolution (g/l) Solution after 197 <0.001 0.0026 0.0012 — — 0.0013 <0.001 0.0013 Neutralization (g/l) Solution after 198 <0.001 <0.001 <0.001 — — 0.0013 <0.001 <0.001 Sulfurization (g/l) Electrowon Metal — <0.1 <0.1 <0.1 36 <0.1 <0.1 0.6 <0.1 (ppm) Cast Indium >99.999 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.2 <0.1 (ppm) (%)

COMPARATIVE EXAMPLE 1

Cast indium was recovered by the same method as that in Example 1, except that grinding and aging were not carried out. In this comparative example, the percentage of coarse particles having a larger particle size than 150 μm was about 86% before dissolution in hydrochloric acid, and the leaching rate was only 81% after 720 minutes at the dissolving step using hydrochloric acid. The amount of tin in the electrowon metal was 0.9 ppm which was larger than that in Examples 1 and 2. Furthermore, the compositions of the solution after dissolution, solution after neutralization and solution after sulfurization, and the grades of the electrowon metal and cast indium, which were obtained in this comparative example, are shown in Table 4, and the relationship between the leaching rate and time in this comparative example is shown in Table 2 and FIG. 2.

TABLE 4 In Sn Cu Pb Na Zn Cr Ni Fe Solution after 164 1.08 0.0025 0.0012 — — 0.0012 0.0010 0.2010 Dissolution (g/l) Solution after 160 <0.001 0.0025 0.0012 — — 0.0012 0.0010 0.0024 Neutralization (g/l) Solution after 160 <0.001 <0.001 <0.001 — — 0.0012 0.0010 <0.001 Sulfurization (g/l) Electrowon Metal — 0.9 <0.1 <0.1 40 <0.1 <0.1 0.7 <0.1 (ppm) Cast Indium >99.999 0.9 <0.1 <0.1 <0.1 <0.1 <0.1 0.5 <0.1 (ppm) (%)

EXAMPLE 3, COMPARATIVE EXAMPLE 2

A dissolving step was carried out by the same method as that in each of Example 1 and Comparative Example 1, except that sulfuric acid was substituted for hydrochloric acid. As a result, the leaching rate was 99% after 240 minutes in Example 3 in which there were used fine particles wherein the percentage of coarse particles having a larger particle size than 150 μm was about 4%, whereas the leaching rate was only 81% after 720 minutes in Comparative Example 2 wherein grinding was not carried out. These results are shown in Table 5.

TABLE 5 Comp. Example 2 Example 3 150 μm<: 86% 150 μm<: 4% Time Leaching Leaching (min.) In(g/l) Rate(%) In(g/l) Rate(%) 0 0 0 0 0 30 69 34 132 65 60 119 59 177 88 90 139 69 189 94 120 144 72 193 96 180 147 73 196 98 240 149 74 198 99 360 153 76 198 99 480 158 79 198 99 600 161 80 198 99 720 162 81 198 99

EXAMPLE 4, COMPARATIVE EXAMPLE 3

A dissolving step was carried out by the same method as that in each of Example 1 and Comparative Example 1, except that nitric acid was substituted for hydrochloric acid. As a result, the leaching rate was 97% after 360 minutes in Example 4 in which there were used fine particles wherein the percentage of coarse particles having a larger particle size than 150 μm was about 4%, whereas the leaching rate was only 79% after 720 minutes in Comparative Example 3 wherein grinding was not carried out. These results are shown in Table 6.

TABLE 6 Comp. Example 3 Example 4 150 μm<: 86% 150 μm<: 4% Time Leaching Leaching (min.) In(g/l) Rate(%) In(g/l) Rate(%) 0 0 0 0 0 30 68 34 129 64 60 117 58 173 86 90 136 67 185 92 120 141 70 189 94 180 143 71 192 95 240 146 73 194 96 360 150 74 194 97 480 154 77 194 97 600 157 78 194 97 720 158 79 194 97 

1. A method for recovering indium, said method comprising: a grinding step of grinding an indium containing substance until the percentage of coarse particles of the indium containing substance is not larger than a predetermined percentage, the coarse particles having a larger particle size than a predetermined particle size; a dissolving step of dissolving fine particles, which are obtained at the grinding step, in an acid; a neutralizing step of adding an alkali to a solution after dissolution, which is obtained at the dissolving step, to neutralize the solution after dissolution so that the solution after dissolution has a pH of 0.5 to 4, to deposit and remove hydroxides of predetermined metal ions in the solution after dissolution; an aging step of aging a solution after neutralization, which is obtained at the neutralizing step, at a predetermined temperature for a predetermined period of time; a sulfurizing step of blowing hydrogen sulfide gas into a solution after aging, which is obtained at the aging step, to deposit and remove sulfides of metal ions; and an electrowinning step of using a solution after sulfurization, which is obtained at the sulfurizing step, as an initial electrolytic solution to electrowin indium metal.
 2. A method for recovering indium as set forth in claim 1, wherein said predetermined particle size is 150 μm.
 3. A method for recovering indium as set forth in claim 1, wherein said predetermined percentage is 10%.
 4. A method for recovering indium as set forth in claim 1, wherein said predetermined temperature is in the range of from a room temperature to 80° C.
 5. A method for recovering indium as set forth in claim 1, wherein said predetermined period of time is not shorter than 2 hours.
 6. A method for recovering indium as set forth in claim 1, which further comprises a classification step of removing coarse particles having a larger particle size than said predetermined particle size by classification before said dissolving step after said grinding step.
 7. A method for recovering indium as set forth in claim 1, wherein said acid is selected from the group consisting of hydrochloric acid, sulfuric acid and nitric acid.
 8. A method for recovering indium as set forth in claim 1, wherein said indium containing substance is an ITO target scrap.
 9. Indium having a purity of not less than 99.999% and containing tin, the content of which is not larger than 0.5 ppm.
 10. Indium obtained by electrowinning which uses indium-tin-oxide as a raw material, and containing tin, the content of which is not larger than 0.5 ppm. 